The present invention relates to a stored program sequence controller, and in particular, to a sequence controller suitable for controlling a system processing both the conditional and sequential control.
In the prior-art stored program sequence controller, there have been mainly utilized two kinds of instruction processing procedures. The first is called a conditional control procedure in which external input signals are scanned for each cycle of a sequence program, and a specific output processing is carried out when the external input signals scanned satisfy a logical condition set by the program, whereas the output processing is skipped and control proceeds to the next step if the condition is not satisfied. This cyclic processing is thus suitable for a conditional control including a complicated random logic.
The second of the procedures is named a sequential control procedure in which a specific output state is retained until a step is completed and control is transferred to the subsequent step when the preceding step is finished, thereby performing a stepwise operation.
That is, the sequential control is utilized to execute step by step a processing based on a signal which confirms a completion of the preceding step.
Since the sequence controller which operates according to the sequential control primarily monitors the transition condition for passing control from a step to another step throughout its operation and an interlock between steps is not thus required, a program can be advantageously constructed simply in accordance with the operation flow of the sequence controller; however, a parallel processing cannot be easily implemented, and there have been difficulties, for example, a special external circuit is necessary to detect an emergency stop condition when executing an interrupt processing for an emergency stop.
On the other hand, although the sequential control can be implemented by the sequence controller operating in accordance with the conditional control procedure, since the output section is set to and remains in the output state only while it is receiving output information from the CPU in the conventional sequence controller system configuration, if the sequence controller is applied to the sequential control, it is necessary to provide a function to keep the output state by its internal processing so as to retain the output state until the transition condition is satisfied as described above. To this end, an interlock between two steps needs to be established in accordance with an internal output, which leads to a disadvantage that the program becomes more complex.